Bell caps are just replacement caps in case your plastic ones break or you want to change things up with different colors.
The "slam caps" that I just posted pics of are a bit different though. Notice how I took the cap off to expose my coil, but my juice is still in there. No filling tool needed, no need to fill from the bottom...just unscrew that cap and wallah, fill it like a Kanger Protank. Tradeoff is less capacity, but, it's so easy to fill now I don't even care.
Ah, that makes sense now, thanks. I'm going to be ordering some of the other kayfun accessories you posted about earlier in the thread... where'd you get that slam cap?